Activation of the ras proto-oncogene has been found in multiple human malignancies and is believed important in many tumorigenesis pathways. We are using a transgenic mouse model which has a structural v-Ha-ras transgene fused to a fetal zeta-globin promoter to study the events of tumorigenesis associated with ras activation. These TG.AC strain transgenic mice rapidly develop large numbers of epidermal papillomas following TPA treatment, some of which progress to malignancy. Histologically, the skin of the TG.AC mice is undistinguishable from that of noncarrier FVB mice and responds with a similar down-regulation of protein kinase C and hyperplasia following tetradecanoylphorbol-13-acetate (TPA) treatment, except for the focal appearance of papillomas on TG.AC skin. Using the techniques of Western blotting and in situ hybridization histochemistry we have localized and quantitated transgenic ras mRNA and protein product expression in epidermal papillomas and are in the process of examining transgene expression in malignancies of the skin and other tissues. Both Western blotting and in situ hybridization histochemistry have demonstrated that the transgene message and protein product are expressed in epidermal papillomas but not in normal or TPA-treated skin. Using in situ hybridization histochemistry experiments the transgene message was further localized to the basal epidermoid cells of the papillomas. Immunohistochemical analysis for PCNA (proliferating cell nuclear antigen) which is a marker for dividing cells localizes proliferating cells of the papilloma to this same basal epidermoid cell population. Thus, using in situ hybridization histochemistry for the transgene message together with immunohistochemical analysis of dividing cells we have shown an association of transgenic ras expression and proliferation in the papillomas. Other experiments with papilloma precursors have suggested that the papillomas develop focally from keratinocytes associated with the hair follicle. Future experiments will examine the role of the hair follicle in papilloma formation and pursue the association of activated ras expression and proliferation.